Fungi are microscopic, spore-bearing eukaryotic organisms. Most fungal species are saprophytes, i.e. organisms that survive on dead organic matter, but some fungal species need living host animals or plants in order to grow. These fungal species may be parasites, which cause disease in the host organism, or they may exist in either a neutral or symbiotic relationship with a host plant.
More than 8,000 fungal species are parasites that can cause disease in plants. Mildew of roses, cereal rust and potato blight are commonly encountered examples of fungal diseases. Plant fungal diseases can have disastrous consequences for humans. For example, in the mid-1800s an epidemic of potato blight in Ireland caused the deaths of a quarter of a million people.
Damping-off and root rot are two fungal diseases that presently cause great economic damage to crop plants. One of the most common causative agents of damping-off is Pythium, a lower fungi and a member of the fungal class Oomycetes. Many other members of Oomycetes, for example, common water mold, potato blight, and downy mildew, are severe plant pathogens. Pythium, as other members of this class, lives in water or on moist soil, and is considered a saprophyte, even though its host organism may be living. Pythium infects crop seedlings at or below the soil surface when, under proper conditions, spores emerge from the Pythium vegetative body.
The disease of damping-off is found in almost every kind of vegetable, flower, cereal and tree and is found all over the world. The symptoms of the disease vary with the developmental stage of the plant when it is infected. In brief, when germinating seeds and seedlings are infected, the infected area becomes discolored and the infected tissue collapses. Older infected plants only have small lesions, but these lesions can girdle the plant and cause death.
Root rot is commonly caused by Rhizoctonia, a higher fungi. This fungus is parasitic on crops such as wheat, corn, stone fruits and forest trees. Rhizoctonia is a member of the subdivision Basidiomycetes, a subdivision of fungi which includes plant rusts, which are among the most destructive of plant diseases. Rusts have caused tremendous losses in wheat, oats, barley, bean, asparagus, cotton, and soybean. Basidiomycetes also includes species that cause smut in corn, oats, wheat, barley and rice.
Rhizoctonia causes root and stem rot on most annual plants throughout the world. The disease symptoms of root and stem rot are similar to those of Pythium-caused damping-off disease. Rhizoctonia exists primarily as a mycelium that consists of long cells with branches that grow at right angles to the main hypha.
The control of Rhizoctonia, Pythium and other plant diseases is a subject of great interest now. Manipulating soil conditions, such as dampness or aeration, is one means used to discourage fungal growth. Drenching the soil with chemicals, such as pentachloronitrobenzene, can reduce damping-off in seed beds. Certain fungicides, including contact and systemic fungicides, are effective. Recently, mulching fields with a layer of rice husks, or with photodegradable plastic, has been tried in an attempt to control Rhizoctonia and Pythium.
There are serious limitations inherent in all these fungal disease control methods. Application of chemical fungicides, even if effective, is expensive. Chemical buildup and runoff can accumulate in the environment and be detrimental to plant and animal life. Plants can become resistant to certain fungicides. Mulching fields is burdensome, and of questionable efficacy.
Currently, there is a great deal of interest in using microbiological control agents (biocontrols) against fungal plant disease. A biocontrol agent is an microorganism that, when added to soil, coated onto seeds, or in some other way introduced to the plant, greatly reduces plant disease incidence and severity. The mode of this disease control is often unknown. It may be because the biocontrol out-competes the pathogen for an ecological niche, because the biocontrol secretes or contains a substance that is toxic to the pathogen, because of parasitism of the biocontrol on the pathogen, or some combination of these or other effects.
Present biocontrols are perceived by some as not effective, reliable, and cost-efficient enough for present large-scale agricultural use. Schroth and Hancock, in a review article on biocontrols, state that "[c]onsiderable work must be done in such areas as developing a highly concentrated inoculum . . . in a form that can be applied commercially." (Schroth and Hancock, "Disease-Suppressive Soil and Root-Colonizing Bacteria," Science 216:1376-1381, June, 1982.)
Several fungal species which are not pathogenic to plants have been found to be effective biocontrol agents. Examples of such fungal biocontrol agents may be found in U.S. Pat. No. 4,828,600 (Trichoderma species in corn) and U.S. Pat. No. 4,259,317 (Pythium species in sugar beets). Fungal species used as biocontrols against fungal pathogens may have advantages over bacteria. For example, a Penicillium species isolated from pea roots has been used to increase pea yield. (Kommedahl, et al. "Variability in Performance of Biological and Fungicidal Seed Treatments in Corn, Peas and Soybeans," Protection Ecology 3:5561, 1981.) Rhizoctonia is known to be controlled by several fungus species, such as species of Trichoderma, Gliocladium and Laetisaria. Biocontrol of plant fungal diseases is promising, but methods for improving the effectiveness of these biocontrol fungi are needed.